Lactide is a well-known intermediate product which can be used in the manufacturing of polymer materials, like polylactic acid (PLA) or PLA-containing copolymers. Lactide (sometimes called dilactide) is a cyclic dimer of lactic acid and is usually manufactured by means of a two-step process. In the first step of this process, lactic acid is polymerized by means of polycondensation into a so-called pre-polymer or oligomer, having a relatively low molecular weight. In the second step, crude lactide is formed from this pre-polymer or oligomer material by means of a so-called ‘backbiting’ process in the presence of a suitable catalyst. The crude lactide material formed in this process can be purified, for example by means of (repeated) crystallization and/or (repeated) distillation. The so-obtained purified lactide may subsequently be used in a polymerization process for the manufacture of PLA or PLA-containing copolymers.
It is well-known that lactide can exist in three different stereochemical structures, which have a diastereomeric relationship. These different structures can be distinguished as R,R-lactide (or D-lactide), S,S-lactide (or L-lactide) and R,S-lactide (or meso-lactide). These three types of lactide are referred to as lactide stereoisomers. Furthermore, a stoichiometric mixture of D- and L-lactide is usually referred to as racemic lactide or rac-lactide. Within the scope of the present invention, the word ‘lactide’ (or dilactide) used in the absence of a prefix (meso-, L-, or D-) generally refers both to any of the three pure lactides (being composed of only one stereoisomer) as well as to mixtures of two or more of the pure lactides.
Knowledge of the purity of the produced lactide is important. This relates to impurities, like free acids (such as lactic acid and lactoyl lactic acid) or water, which impurities may have a strong influence on a lactide-to-PLA polymerization process. This also relates to the so-called stereochemical purity of the produced lactide material. The relative amount(s) of the before-mentioned lactide enantiomer(s) in a final polymer-grade lactide depends firstly on the stereochemical purity of the lactic acid used as starting material in the production of lactide. Secondly, undesired lactide enantiomer(s) can also be formed during the lactide production process itself as a result of racemization.
The amount of meso-lactide will increase if only one of the two stereo-centers in pure L-lactide or pure D-lactide racemases (epimerization). Finally, the details of the purification processes downstream of the production process will determine the stereochemical composition of the product stream(s).
Currently, chromatographic methods are still used in order to determine or quantify the amount of meso-lactide in a composition containing meso-lactide and at least one other lactide enantiomer with respect of the total amount of lactide enantiomers. In practice these methods require tedious sample preparation and work-up of lactide-containing material as well as chromatographic analysis in order to determine the exact amount of meso-lactide in the composition.
According to the experience of the inventors, the known chromatographic methods for quantification of meso-lactide in the total amount of lactide enantiomers are time-consuming and labor-intensive in their use. Moreover, the results of the known quantification methods are not immediately available.
Therefore, determination of meso-lactide by means of the known separation methods has the drawbacks of being less suitable to monitor the lactide quality online under mass production circumstances.